Why does this code end up in an infinite loop, reading from std::cin

Question

Hi I try to create a input function for my function through vector.

However, I don't know why my input become infinite loop?

do {            
    cout << "Please enter the next number: ";
    cin >> num;
    number.push_back(num);
    cout << "Do you want to end? enter 0 to continue.";
    dec = NULL;
    cin >> dec;
} while(dec == 0);

if dec is not a pointer then you should not use `dec = NULL;`. If you cant replace `NULL` with `nullptr` your code should be considered broken. — NathanOliver, Apr 14 '15 at 16:59
You should [add some concrete inputs](http://stackoverflow.com/posts/29633122/edit) given with your question, when your code shows this behavior. — πάντα ῥεῖ, Apr 14 '15 at 17:28
To give more explanation why I've rolled back your question to it's original state, please follow the according discussion on [Meta SO](http://meta.stackoverflow.com/questions/290240/should-i-rollback-the-ops-edits-if-they-just-replicate-my-answers-proposed-ch). — πάντα ῥεῖ, Apr 14 '15 at 21:50

score 1 · Accepted Answer · edited May 23 '17 at 11:43

"I don't know why my input become infinite loop."

The only reason I can imagine is, that any incorrect input sets cin to fail state. In this case (e.g. an invalid number was entered, or just ENTER was pressed) cin is set to fail state and your value in dec won't change evermore. Once cin is in fail state any subsequent input operations will fail respectively, and the subject for input won't be changed.

To be proof against such behavior, you have to clear() the std::istream's state, and read up to a safe point, before proceeding (see also: How to test whether stringstream operator>> has parsed a bad type and skip it):

do {
    cout << "Please enter the next number: ";
    if(cin >> num) {
        number.push_back(num);
    }
    else {
       cerr << "Invalid input, enter a number please." << std::endl;
       std::string dummy;  
       cin.clear();
       cin >> dummy;
    }
    cout << "Do you want to end? enter 0 to continue.";
    dec = -1;
    if(!(cin >> dec)) {
       std::string dummy;  
       cin.clear();
       cin >> dummy;
       break; // Escape from the loop if anything other than 0 was
              // typed in
    }
} while(dec == 0);

Here are three working demos with different inputs to end the loop:

1st Input:

ENTER

2nd Input:

3rd Input

The loop is finite, and output for all of the above is

_{You should also note that I have changed bool dec; to int dec;, but that's probably a minor point.}

@HuangElizabeth _"next line of code"_ I don't get what you mean? — πάντα ῥεῖ, Apr 14 '15 at 17:23
There is a loop for output after my while function. If I press 0 to end the program. The program will not go to next loop for output. I have edit my post. — Huang Elizabeth, Apr 14 '15 at 17:29
@HuangElizabeth Please have a look at my working demos at ideone. — πάντα ῥεῖ, Apr 14 '15 at 17:53
@HuangElizabeth _"The program works now "_ You may consider accepting this answer by clicking the big white check mark at the left side. — πάντα ῥεῖ, Apr 14 '15 at 19:12

Why does this code end up in an infinite loop, reading from std::cin

1 Answers1